The oil extension of hydrocarbon elastomer compositions is a practice used by manufacturers of such compositions and by fabricators of elastomeric products to lower durometer, improve processability, permit higher filler loadings, and decrease pound volume cost. This practice is also utilized to improve tear resistance over a range of filler loadings, to improve low temperature properties, and in the case of automobile tires, to improve the wear resistance.
The addition of silicone oils containing various functional groups, such as silanol, methoxy, amino, etc., to amorphous high molecular weight silicone elastomrs reinforced with high surface area fillers, as process aids, has been practiced to improve processability and to optimize properties of cured reinforced silicone elastomers. Oil extension of amorphous high molecular weight silicone elastomer compositions has not been practiced because of the severe reduction in the level of physical properties of the cured silicone elastomer compositions caused by even small amounts of such extender oils.
Merker teaches in U.S. Pat. No. 3,202,634 crystalline silarylenesiloxane block copolymers and condensation-curable compositions based thereon. At Column 8, line 69, and following he refers to the peroxide cure of variants of these copolymers containing alkenyl groups. However, following the brief teaching gives a cured polymer with such a tight cure that formation of the crystallites necessary for high strength properties of the cured composition is impeded.
Bobear discloses in U.S. Pat. No. 3,660,345 the addition of vinyl-on-chain sources for controlling the cross-link density of amorphous silicone elastomer compositions.
A silarylenesiloxane copolymer, crystalline in nature at ambient room temperature, with controllable cross-links density would provide a balance of high strength and toughness not found in the compositions of the prior art.
To make wire and cable jacketing or insulation from such compositions, the compositions are extruded on wire at temperatures sufficient to form a homogeneous melt, i.e., no crystallites, and the resultant extrudate is then cured by continuous vulcanization at 207.degree. C. (maximum steam) with a 1-11/2 minute residence time. An extruder barrel temperature of 110.degree. C. (minimum) is necessary to provide the melt. The alkyl peroxide catalysts presently available having the highest activation temperatures and which provide the desired vulcanizate properties after 1-11/2 minutes at 207.degree. C. are, dicumylperoxide, with a 10 hour half-cure time at 115.degree. C. and .alpha.,.alpha.'-bis(t-butylperoxy)diisopropylbenzene with a 10 hour half-cure time at 122.degree. C. Unfortunately, the melt viscosity of the tough rubber compounds based upon silphenylene at these processing temperatures is somewhat high, and consequently, the heat-of-shear generated on mixing results in an internal extruder barrel temperature which is sufficient to activate the peroxides resulting in premature curing or scorching. The magnitude of the scorch problem is not fully realized until processing is carried out with a standard 3-inch extruder equipped with a 3:1 compression screw typically used for thermoplastics. The scorched material yields a grainy exterior surface demonstrating poor knitting which renders the product unacceptable.
The processability of these tough rubber compounds based on silphenylene via extrusion where the stock must be chemically cured, becomes critical due to the narrow processing temperature range between the extruder barrel temperature necessary to obtain a homogeneous melt and the temperature at which the peroxide catalysts become activated. Temperatures below this range result in an incomplete melt, and consequently, the ingredients are poorly mixed, and temperatures above this range result in premature curing or scorching of the product. It can be readily realized that it is desirable to expand this processing temperature range and to provide processability of the tough rubber compounds based on silphenylene, at temperatures above and below the currently recognized ranges. Under some circumstances, it may also be desirable to produce highly loaded compounds, i.e. high filler content, with good processability at substantially lower pound per volume costs.